If you are a one class entry this is very easy.
It's just a question of renaming your groups, then your parents will move with the newly named group.
1. Go to the dashboard and click "add/Edit groups"
2. Then go to Year/Dosbarth 6, click on it and rename that Year/Dosbarth 6 Alumni, something like that.
NB: When you change the name of the groups, You will have to save changes, then confirm that you are moving subscribers with that new group. Then PLEASE remember to click save changes again.
3. Then rename Year/Dosbarth 5 to Year/Dosbarth 6, etc etc.
4. Create a new group for Nursery. If your Nursery is splitting, you can now go to the subscribers tab at the top of the screen, search for Reception, take out your email address, and move those staying in Nursery back into the new nursery group.
5. Once you are happy with everything you can then delete Year/Dosbarth 6 Alumni.
I always do it that way first, just so you don't mess things up and delete the wrong group accidentally and lose your way when changing groups.
I've done it before and it's not a pretty site.